Etching apparatus and etching method

ABSTRACT

An etching apparatus includes: a placement table serving as a lower electrode and configured to place a workpiece to be subjected to an etching processing thereon; a DC power supply configured to generate a negative DC voltage applied to the placement table; and a controller configured to: periodically apply a negative DC voltage to the placement table from the DC power supply when the etching processing on the workpiece placed on the placement table is initiated, and decrease a frequency of the negative DC voltage applied to the placement table with an elapse of processing time of the etching processing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority from Japanese PatentApplication No. 2018-087214, filed on Apr. 27, 2018 with the JapanPatent Office, the disclosure of which is incorporated herein in itsentirety by reference.

TECHNICAL FIELD

The present disclosure relates to an etching apparatus and an etchingmethod.

BACKGROUND

An etching apparatus that performs an etching processing on a workpiece(e.g., a semiconductor wafer) using plasma has been known in the relatedart. Such an etching processing apparatus includes a placement tablewithin a processing container that is designed to be capable of forminga vacuum space therein. The placement table is configured to place aworkpiece therein and also functions as an electrode of the etchingapparatus. In the etching processing apparatus, an etching processingusing plasma is performed on a workpiece placed on the placement tableby applying, for example, a predetermined radio-frequency power to theplacement table. In addition, in the etching apparatus, radio-frequencypower may be applied to the placement table for bias purpose when theetching processing is performed using plasma. By applyingradio-frequency power to the placement table for bias purpose, the ionsin the plasma are drawn into the workpiece, and holes and grooves areefficiently formed in the workpiece.

In recent years, an aspect ratio required for holes or grooves formed ina workpiece by an etching processing is increasing. In an etchingprocessing of holes or grooves having a high aspect ratio, as theetching processing progresses, since the rectilinearity of the ionsdrawn into the workpiece decreases, etching characteristics deteriorate.It is known that the rectilinearity of the ions drawn into the workpiecebecomes higher as the frequency of the radio-frequency power for biasapplied to the placement table becomes lower.

Thus, there has been known a technique in which a plurality ofradio-frequency power supplies for generating a plurality ofradio-frequency powers for bias having different frequencies are mountedin an etching apparatus and the frequency of the radio-frequency powerfor bias is changed by selectively switching connection between theplurality of radio-frequency power supplies and the placement tableduring the etching processing. See, for example, Japanese PatentLaid-open Publication No. 2008-053516.

SUMMARY

An etching apparatus according to an aspect of the present disclosureincludes: a placement table serving as a lower electrode and configuredto place a workpiece to be subjected to an etching processing; a DCpower supply configured to generate a negative DC voltage applied to theplacement table; and a controller configured to: periodically apply anegative DC voltage to the placement table from the DC power supply whenthe etching processing on the workpiece placed on the placement table isinitiated, and decrease a frequency of the negative DC voltage appliedto the placement table with an elapse of processing time of the etchingprocessing.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a schematicconfiguration of an etching apparatus according to a first embodiment.

FIG. 2 is a view schematically illustrating the state of ions when anetching processing is started.

FIG. 3 is a view schematically illustrating the state of ions when anetching processing proceeds.

FIG. 4 is a flowchart illustrating an example of a flow of an etchingmethod according to an embodiment.

FIG. 5 is a view illustrating an example of a DC frequency decreasemode.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawing, which form a part hereof. The illustrativeembodiments described in the detailed description, drawing, and claimsare not meant to be limiting. Other embodiments may be utilized, andother changes may be made without departing from the spirit or scope ofthe subject matter presented here.

Hereinafter, various embodiments will be described in detail withreference to the drawings. In each of the drawings, the same orcorresponding components will be denoted by the same reference numerals.

An etching apparatus that performs an etching processing on a workpiece(e.g., a semiconductor wafer) using plasma has been known in the relatedart. Such an etching processing apparatus includes a placement tablewithin a processing container that is designed to be capable of forminga vacuum space therein The placement table is configured to place aworkpiece therein and also functions as an electrode of the etchingapparatus. In the etching processing apparatus, an etching processingusing plasma is performed on a workpiece placed on the placement tableby applying, for example, a predetermined radio-frequency power to theplacement table. In addition, in the etching apparatus, radio-frequencypower may be applied to the placement table for bias purpose when theetching processing is performed using plasma. By applyingradio-frequency power to the placement table for bias purpose, the ionsin the plasma are drawn into the workpiece, and holes and grooves areefficiently formed in the workpiece.

In recent years, an aspect ratio required for holes or grooves formed ina workpiece by an etching processing is increasing. In an etchingprocessing of holes or grooves having a high aspect ratio, as theetching processing progresses, since the rectilinearity of the ionsdrawn into the workpiece decreases, etching characteristics deteriorate.It is known that the rectilinearity of the ions drawn into the workpiecebecomes higher as the frequency of the radio-frequency power for biasapplied to the placement table becomes lower.

Thus, there has been known a technique in which a plurality ofradio-frequency power supplies for generating a plurality ofradio-frequency powers for bias having different frequencies are mountedin an etching apparatus and the frequency of the radio-frequency powerfor bias is changed by selectively switching connection between theplurality of radio-frequency power supplies and the placement tableduring the etching processing.

However, when the plurality of radio-frequency power supplies aremounted in the etching apparatus, the apparatus configuration of theetching apparatus becomes complicated. For this reason, it is expectedthat the deterioration of etching characteristics is suppressed with asimple apparatus configuration.

First Embodiment

[Configuration of Etching Apparatus]

FIG. 1 is a schematic cross-sectional view illustrating a schematicconfiguration of an etching apparatus 10 according to a firstembodiment. The etching apparatus 10 includes a processing container 12.The processing container 12 has a substantially cylindrical shape. Theprocessing container 12 defines a processing space in which plasma isgenerated. The processing container 12 is made of, for example,aluminum. The processing container 12 is connected to a groundpotential. On the inner wall surface of the processing container 12, aplasma-resistant film is formed. The film may be a film formed by ananodic oxidation processing or a ceramic film such as, for example, afilm formed of yttrium oxide. In addition, a passage 12 p is formed inthe side wall of the processing container 12. When a wafer W as theworkpiece is loaded into the processing container 12 and when the waferW is unloaded from the processing container 12, the wafer W passesthrough the passage 12 p. In order to open and close the passage 12 p, agate valve 12 g is provided along the side wall of the processingcontainer 12.

In the processing container 12, a support unit 15 extends upward fromthe bottom portion of the processing container 12. The support unit 15has a substantially cylindrical shape, and is formed of an insulatingmaterial such as ceramics. A placement table 16 is mounted on thesupport unit 15. The placement table 16 is supported by the support unit15. The placement table 16 is configured to support a wafer W within theprocessing container 12. The placement table 16 includes a base 18 andan electrostatic chuck 20. The base 18 is made of a conductive materialsuch as, for example, aluminum, and has a substantially disk shape. Thebase 18 has a function as a lower electrode.

In the base 18, a flow path 18 f is provided. The flow path 18 f is aflow path for a heat exchange medium. As the heat exchange medium, aliquid coolant or a coolant for cooling the base 18 by vaporizationthereof (e.g., fluorocarbon) is used. The heat exchange medium issupplied to the flow path 18 f from a chiller unit provided outside theprocessing container 12 through a pipe 23 a. The heat exchange mediumsupplied to the flow path 18 f is returned to the chiller unit through apipe 23 b. That is, the heat exchange medium is supplied so as tocirculate between the flow path 18 f and the chiller unit.

The electrostatic chuck 20 is provided on the base 18. The electrostaticchuck 20 includes a main body formed of an insulator and a film-shapedelectrode provided inside the main body. A DC power supply iselectrically connected to the electrode of the electrostatic chuck 20.When the voltage is applied from the DC power supply to the electrode ofthe electrostatic chuck 20, an electrostatic attractive force isgenerated between the wafer W disposed on the electrostatic chuck 20 andthe electrostatic chuck 20. Due to the generated electrostaticattractive force, the wafer W is attracted to the electrostatic chuck20, and held by the electrostatic chuck 20. A focus ring FR is disposedon the peripheral edge region of the electrostatic chuck 20. The focusring FR has a substantially annular plate shape, and is formed of, forexample, silicon. The focus ring FR is disposed so as to surround theedge of the wafer W.

The etching apparatus 10 is provided with a gas supply line 25. The gassupply line 25 supplies a heat transfer gas such as, for example, Hegas, from the gas supply mechanism to a space between the upper surfaceof the electrostatic chuck 20 and the rear surface (lower surface) ofthe wafer W.

A cylindrical portion 28 extends upward from the bottom portion of theprocessing container 12. The cylindrical portion 28 extends along theouter periphery of the support unit 15. The cylindrical portion 28 isformed of a conductive material, and has a substantially cylindricalshape. The cylindrical portion 28 is connected to a ground potential. Aninsulating unit 29 is provided on the cylindrical portion 28. Theinsulating unit 29 has an insulating property, and is formed of, forexample, quartz or ceramics. The insulating unit 29 extends along theouter periphery of the placement table 16.

The etching apparatus 10 further includes an upper electrode 30. Theupper electrode 30 is provided above the placement table 16. The upperelectrode 30 closes the upper opening of the processing container 12together with a member 32. The member 32 has an insulating property. Theupper electrode 30 is supported in the upper portion of the processingcontainer 12 via the member 32. When a radio-frequency power supply 62to be described later is electrically connected to the base 18, theupper electrode 30 is connected to the ground potential.

The upper electrode 30 includes a top plate 34 and a support 36. Thelower surface of the top plate 34 faces the processing space. The topplate 34 is provided with a plurality of gas ejection holes 34 a. Eachof the plurality of gas ejection holes 34 a penetrates the top plate 34in the plate thickness direction (the vertical direction). The top plate34 is formed of, for example, silicon, although it is not limitedthereto. Alternatively, the top plate 34 may have a structure in which aplasma-resistant film is provided on the surface of a base material madeof aluminum. The film may be a film formed by an anodic oxidationprocessing or a ceramic film such as, for example, a film formed ofyttrium oxide.

The support 36 is a component that detachably supports the top plate 34.The support 36 may be formed of a conductive material such as, forexample, aluminum. A gas diffusion chamber 36 a is provided inside thesupport 36. A plurality of gas holes 36 b extend downward from the gasdiffusion chamber 36 a. The plurality of gas holes 36 b communicate withthe plurality of gas ejection holes 34 a, respectively. The support 36is provided with a gas inlet 36 c configured to guide a processing gasto the gas diffusion chamber 36 a, and a gas supply pipe 38 is connectedto the gas inlet 36 c.

To the gas supply pipe 38, a gas source group 40 is connected through avalve group 42 and a flow rate controller group 44. The gas source group40 includes a plurality of gas sources. The valve group 42 includes aplurality of valves, and the flow rate controller group 44 includes aplurality of flow rate controllers. Each of the plurality of flow ratecontrollers of the flow rate controller group 44 is a mass flowcontroller or a pressure control-type flow rate controller. Each of theplurality of gas sources of the gas source group 40 is connected to thegas supply pipe 38 through a corresponding valve of the valve group 42and a corresponding flow rate controller of the flow rate controllergroup 44. The etching apparatus 10 is capable of supplying a gas from atleast one source selected among the plurality of gas sources of the gassource group 40 into the processing container 12 at an individuallyadjusted flow rate.

A baffle plate 48 is provided between the cylindrical portion 28 and theside wall of the processing container 12. The baffle plate 48 may beconstituted, for example, by coating an aluminum base material with aceramic such as, for example, yttrium oxide. A large number of throughholes are formed in the baffle plate 48. Under the baffle plate 48, anexhaust pipe 52 is connected to the bottom portion of the processingcontainer 12. An exhaust device 50 is connected to the exhaust pipe 52.The exhaust device 50 has a pressure controller such as, for example, anautomatic pressure control valve, and a vacuum pump such as, forexample, a turbo molecular pump, so that the processing container 12 ccan be decompressed.

As illustrated in FIG. 1, the etching apparatus 10 further includes aradio-frequency power supply 62. The radio-frequency power supply 62generates radio-frequency power for generating plasma by exciting thegas within the processing container 12. The radio-frequency power forplasma generation has a frequency within a range of 27 to 100 MHz, forexample, a frequency of 60 MHz. The radio-frequency power supply 62 isconnected to the base 18 through a matching circuit 64. The matchingcircuit 64 is a circuit configured to match the output impedance of theradio-frequency power supply 62 and the load side (base 18 side)impedance.

The etching apparatus 10 further includes a DC power supply 70. The DCpower supply 70 generates a negative DC voltage (hereinafter, properlyto as a “DC voltage” as appropriately) to be applied to the placementtable 16 (the base 18). The DC power supply 70 is, for example, amonopolar-type variable DC power supply. The DC power supply 70 may be abipolar-type variable DC power supply. The DC power supply 70 iselectrically connected to the base 18 through a low-pass filter (LPF)72. The DC voltage generated in the DC power supply 70 is used as a biasvoltage for drawing ions in the plasma into the wafer W placed on theplacement table 16. In addition, the DC power supply 70 is configured tobe capable of changing the frequency (hereinafter, referred to as a “DCfrequency” as appropriate) of the generated DC voltage under the controlof a controller 95 to be described later. In addition, the DC powersupply 70 is configured to be capable of changing the absolute value ofthe generated DC voltage under the control of the controller 95.

The etching apparatus 10 further includes the controller 95. Thecontroller 95 includes a CPU (Central Processing Unit) and controls eachunit of the etching apparatus 10. The controller 95 is connected with auser interface 96 including a keyboard on which a process managerperforms an input operation of commands or the like to manage theetching apparatus 10 and a display on which the operating status of theetching apparatus 10 is visualized and displayed.

In addition, the controller 95 is connected with a storage unit 97 thatstores a control program for realizing various processes executed by theetching apparatus 10 under the control of the controller 95, and arecipe stored with, for example, processing condition data. The storageunit 97 is, for example, a hard disk or a semiconductor memory. Further,the storage unit 97 may be a portable storage medium readable by acomputer. In this case, the controller 95 acquires, for example, acontrol program stored in the storage medium via a device that readsdata from the storage medium. The storage medium is, for example, aCD-ROM or a DVD.

The controller 95 controls each unit of the etching apparatus 10 andexecutes various processings in the etching apparatus 10 by reading anarbitrary recipe from the storage unit 97 and executing the recipe inresponse to, for example, an instruction from the user made via the userinterface 96. For example, the controller 95 causes the etchingapparatus 10 to perform an etching processing of forming holes orgrooves on a wafer W by plasma.

In recent years, an aspect ratio required for holes or grooves formed ina wafer W by an etching processing is increasing. In the etchingprocessing of holes or grooves having a high aspect ratio, as theetching processing progresses, the rectilinearity of the ions drawn intothe workpiece decreases, and thus etching characteristics deteriorate.

FIG. 2 is a view schematically illustrating the state of ions when anetching processing is started. For example, as illustrated in FIG. 2,since the depth of a hole h formed in a wafer W when an etchingprocessing is started is relatively shallow, the rectilinearity of ionsin the hole h is maintained.

Meanwhile, when the etching processing proceeds, etching characteristicsdeteriorate. FIG. 3 is a view schematically illustrating the state ofions when an etching processing proceeds. For example, as illustrated inFIG. 3, as the etching progresses and the depth of the hole h becomesdeeper, the rectilinearity of ions decreases in the hole h. Since therectilinearity of ions decreases in the hole h as described above, theetching characteristics deteriorate. For example, since it is difficultfor ions to reach the bottom of the hole h, an etching rate is lowered.Further, for example, since the side wall of the hole h is scraped morethan necessary, defects in etching shape such as, for example, bendingand bowing occur. In the case where radio-frequency power for bias isapplied to the placement table 16, it is known that the rectilinearityof ions drawn into a wafer W increases as the frequency of theradio-frequency power for bias applied to the placement table 16decreases. Therefore, in the case where a DC voltage is applied to theplacement table 16 instead of the radio-frequency power for bias, it isbelieved that it is possible to suppress the rectilinearity of ions fromdeteriorating by lowering the frequency of the DC voltage applied to theplacement table 16.

Therefore, when the etching processing on the wafer W is initiated, thecontroller 95 periodically applies a DC voltage from the DC power supply70 to the placement table 16, and decreases the frequency of the DCvoltage (i.e., the DC frequency) applied to the placement table 16 withthe elapse of the processing time of the etching processing.

[Flow of Etching Method]

Next, a flow of an etching method executed by the etching apparatus 10according to the present embodiment will be described. FIG. 4 is aflowchart illustrating an example of a flow of an etching methodaccording to an embodiment. FIG. 5 is a view illustrating an example ofa DC frequency decrease mode. Before the initiation of the flowchartillustrated in FIG. 4, a wafer W is loaded into the processing container12, the loaded wafer W is placed on the placement table 16, a processinggas is supplied into the processing container 12, and the pressurewithin the processing container 12 is controlled to a predeterminedpressure.

First, the controller 95 controls the DC power supply 70 to periodicallyapply a DC voltage from the DC power supply 70 to the placement table 16(S101, time point t₁ in FIG. 5). Hereinafter, it is assumed that thelength of the period during which the DC voltage is applied to theplacement table 16 is T_(on) and the length of the period during whichthe application of the DC voltage is stopped is T_(off).

Subsequently, the controller 95 controls the radio-frequency powersupply 62 to apply radio-frequency power of a predetermined frequency tothe placement table 16 (time point t₂ in FIG. 5). As a result, plasma ofthe processing gas is generated in the processing container 12, and theetching processing for the wafer W is initiated by the generated plasma(S102).

Subsequently, while the etching processing is proceeding (that is,during time points t₂ to t₃ in FIG. 5), the controller 95 controls theDC power supply 70 to decrease the frequency of the DC voltage appliedto the placement table 16 (i.e., the DC frequency) (S103, time points t₂to t₃ in FIG. 5). The DC frequency is represented by 1/(T_(on)+T_(off)).

In the present embodiment, the controller 95 continuously decreases theDC frequency with the elapse of the processing time of the etchingprocessing from time points t₂ to t₃. As a result, even when the etchingprocessing proceeds and the depth of the hole formed in the wafer Wbecomes deeper, the deterioration of the rectilinearity of the ions inthe hole is suppressed.

Subsequently, the controller 95 controls the radio-frequency powersupply 62 to stop the application of the radio-frequency power at timepoint t₃ when the hole having a predetermined depth is formed in thewafer W, and controls the DC power supply 70 to stop the application(time point t₃ in FIG. 5). Then, the supply of the processing gas isstopped, the processing container 12 is evacuated, and the wafer W isunloaded from the processing container 12. As a result, the etchingprocessing for the wafer W is terminated (S104).

As described above, the plasma processing apparatus 10 according to anembodiment includes the placement table 16, the DC power supply 70, andthe controller 95. A wafer W to be subjected to a plasma etching isplaced on the placement table 16, and the placement table 16 serves as alower electrode. The DC power supply 70 generates a negative DC voltageto be applied to the placement table 16. The controller 95 periodicallyapplies a negative DC voltage to the placement table 16 from the DCpower supply 70 when the etching processing for the wafer W placed onthe placement table 16 is initiated. Then, the controller 95 decreasesthe frequency of the negative DC voltage applied to the placement table16 with the elapse of the processing time of the etching processing. Asa result, in the etching apparatus 10, even in the case where the depthof a hole or a groove formed in the wafer W becomes deeper as theetching processing proceeds, it is possible to suppress thedeterioration of the rectilinearity of ions in the hole or the groove,and to suppress the deterioration of etching characteristics. Forexample, the energy of ions reaching the bottom of the hole or thegroove increases, and the deterioration of the etching rate issuppressed. In addition, for example, etching of the side wall of a holeor a groove is suppressed, and a defect in an etching shape such as, forexample, bending or bowing are suppressed.

Here, in order to suppress the deterioration in the rectilinearity ofions, it may be considered that a plurality of radio-frequency powersupplies for generating a plurality of radio-frequency powers for biashaving different frequencies are mounted in the etching apparatus, andthat connection between the plurality of radio-frequency power suppliesand the placement table 16 is selectively switched during the etchingprocessing. However, when the plurality of radio-frequency powersupplies are mounted in the etching apparatus, the apparatusconfiguration of the etching apparatus becomes complicated. In thisregard, in the etching apparatus 10 according to an embodiment, it ispossible to suppress deterioration in the rectilinearity of ions withoutmounting a plurality of radio-frequency power supplies in the etchingapparatus 10. Consequently, in the etching apparatus 10, it is possibleto suppress the deterioration of etching characteristics with a simpleapparatus configuration in which the frequency of the negative DCvoltage periodically applied from the DC power supply 70 to theplacement table 16 is decreased with the elapse of the processing timeof the etching processing.

In the above-described embodiments, the controller 95 continuouslylowers the frequency of the negative DC voltage applied to the placementtable 16 with the elapse of the processing time of the etchingprocessing, but the disclosed technology it is not limited thereto. Forexample, the controller 95 may gradually lower the frequency of thenegative DC voltage applied to the placement table 16 with the elapse ofthe processing time of the etching processing. The time widths ofrespective steps of decreasing the frequency of the negative DC voltageapplied to the placement table 16 may be the same or different. Further,the controller 95 may decrease the frequency of the negative DC voltageapplied to the placement table 16 from a first frequency to a secondfrequency lower than the first frequency at a time point when apredetermined time has elapsed since the etching processing wasinitiated.

In addition, in the above-described etching apparatus 10, the controller95 may increase or decrease the absolute value of the negative DCvoltage applied to the placement table 16 with the elapse of theprocessing time of the etching processing. For example, when thefrequency of the negative DC voltage applied to the placement table 16from the DC power supply 70 decreases with the elapse of the processingtime of the etching processing, the frequency of switching on and off ofthe DC power supply 70 decreases, and thus the amount of heat generatedby the DC power supply 70 decreases. In this case, the controller 95controls the DC power supply 70 to increase the absolute value of thenegative DC voltage applied to the placement table 16 with the elapse ofthe processing time of the etching processing. As a result, in theetching apparatus 10, it is possible to further suppress deteriorationof etching characteristics even in the case where the etching processingprogresses and the depth of the hole or the groove formed in the wafer Wbecomes deeper. For example, by increasing the absolute value of thenegative DC voltage applied to the placement table 16, the number ofions reaching the bottom of the hole or the groove is increased, and thedeterioration of the etching rate is suppressed.

When the absolute value of the negative DC voltage applied to theplacement table 16 is increased, positive ions in the plasma are drawninto the wafer W, whereby the potential of the plasma is lowered. Whenthe potential of the plasma is lowered, the electric potential of thewafer W fluctuates in order to maintain the electrical neutralitybetween the plasma and the wafer W, and thus the potential differencebetween the wafer W and the placement table 16 on which the wafer W isplaced increases. As a result, discharge may occur between the placementtable 16 and the wafer W in some cases. Therefore, in the etchingapparatus 10, the control of the DC power supply 70 may be performeddepending on the potential difference between the placement table 16 andthe wafer W. For example, in the etching apparatus 10, the controller 95calculates the potential difference between the placement table 16 andthe wafer W after increasing the absolute value of the negative DCvoltage applied to the placement table 16. For calculating the potentialdifference between the placement table 16 and the wafer W, for example,the voltage of the wafer W measured by a measurement device disposed inthe vicinity of the focus ring FR surrounding the wafer W is used. Then,when the calculated potential difference exceeds a predeterminedthreshold, the controller 95 controls the DC power supply 70 so as toshift the value of the negative DC voltage applied to the placementtable 16 by a shift amount for decreasing the potential difference. Thismakes it possible to suppress the occurrence of discharge between theplacement table 16 and the wafer W.

According to the present disclosure, it is possible to suppress thedeterioration of etching characteristics with a simple apparatusconfiguration.

From the foregoing, it will be appreciated that various embodiments ofthe present disclosure have been described herein for purposes ofillustration, and that various modifications may be made withoutdeparting from the scope and spirit of the present disclosure.Accordingly, the various embodiments disclosed herein are not intendedto be limiting, with the true scope and spirit being indicated by thefollowing claims.

What is claimed is:
 1. An etching apparatus comprising: a placementtable serving as a lower electrode and configured to place a workpieceto be subjected to an etching processing thereon; a DC power supplyconfigured to generate a negative DC voltage applied to the placementtable; and a controller configured to: periodically apply a negative DCvoltage to the placement table from the DC power supply when the etchingprocessing on the workpiece placed on the placement table is initiated;and decrease a frequency of the negative DC voltage applied to theplacement table with an elapse of processing time of the etchingprocessing.
 2. The etching apparatus according to claim 1, wherein thecontroller decreases the frequency of the negative DC voltage applied tothe placement table continuously or stepwise with an elapse ofprocessing time of the etching processing.
 3. The etching apparatusaccording to claim 1, wherein the controller decreases the frequency ofthe negative DC voltage applied to the placement table from a firstfrequency to a second frequency lower than the first frequency at a timepoint when a predetermined time has elapsed after initiation of theetching processing.
 4. The etching apparatus according to claim 2,wherein the controller increases or decreases an absolute value of thenegative DC voltage applied to the placement table with the elapse ofthe processing time of the etching processing.
 5. The etching apparatusaccording to claim 4, wherein the controller calculates a potentialdifference between the placement table and the workpiece afterincreasing the absolute value of the negative DC voltage applied to theplacement table, and when the potential difference exceeds apredetermined threshold, the controller controls the DC power supply soas to shift a value of the negative DC voltage applied to the placementtable by a shift amount for decreasing the potential difference.
 6. Theetching apparatus according to claim 1, wherein the controller increasesor decreases an absolute value of the negative DC voltage applied to theplacement table with the elapse of the processing time of the etchingprocessing.
 7. The etching apparatus according to claim 1, wherein thecontroller calculates a potential difference between the placement tableand the workpiece after increasing the absolute value of the negative DCvoltage applied to the placement table, and when the potentialdifference exceeds a predetermined threshold, the controller controlsthe DC power supply so as to shift a value of the negative DC voltageapplied to the placement table by a shift amount for decreasing thepotential difference.
 8. An etching method comprising: periodicallyapplying a negative DC voltage to a placement table serving as a lowerelectrode and configured to place a workpiece thereon from a DC powersupply configured to generate the negative DC voltage applied to theplacement table when an etching processing is initiated on the workpieceplaced on the placement table; and decreasing a frequency of thenegative DC voltage applied to the placement table from the DC powersupply with an elapse of processing time of the etching processing.